Improve dominator computation to avoid worst-case quadratic time.

Improve dominator computation to avoid worst-case quadratic time.

In case of a degenerated CFG like in the example below processing
predecessors in the wrong order yields n^2 runtime.

  do {
    if (x) break;
    if (x) break;
    if (x) break;
    if (x) break;
    if (x) break;
    if (x) break;
    if (x) break;
    if (x) break;
    if (x) break;
    if (x) break;
    if (x) break;
    // etc.
  } while (false);

Reversing iteration order avoids this.
Committed: http://code.google.com/p/v8/source/detail?r=9905

[floitsch, 2011-11-08 09:26:23]

LGTM.